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1C 9069 



Bureau of Mines Information Circular/1986 



Aluminum Fluxing Salts: A Critical Review 
of the Chemistry and Structures of Alkali 
Aluminum Halides 

By Charles A. Sorrell, John G. Groetsch, Jr., and D. M. Soboroff 




UNITED STATES DEPARTMENT OF THE INTERIOR 



f^t^ J &ti £u/ , faiAjxjxuc <f fiAt*) 



Information Circular 9069 

H / 



Aluminum Fluxing Salts: A Critical Review 
of the Chemistry and Structures of Alkali 
Aluminum Halides 



By Charles A. Sorrell, John G. Groetsch, Jr., and D. M. Soboroff 




UNITED STATES DEPARTMENT OF THE INTERIOR 

Donald Paul Hodel, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 





UNIT OF MEASURE ABBREVIATIONS USED IN 


THIS REPORT 


A 


angstrom 


h 


hour 


°C 


degree Celsius 


mol pet 


mole percent 


g 


gram 


wt pet 


weight percent 


g/cm 3 


gram per cubic 
centimeter 









0<> « 



Library of Congress Cataloging in Publication Data: 



Sorrel!, Charles A 








• 


Aluminum fluxing salts. 










(Bureau of Mines information circular ; 


9069) 






Bibliography: p. 34-37. 










Supt. of Docs, no.: I 28.27 


: 9069. 








1. Aluminum— Metallurgy. 


2. Alkali 


aluminum ha 


lides. 


3. Flux 


(Metallurgy). I. Groetsch, J. 


G. (John G. 


), II. Soboro 


ff, D. 


M. (David 


M.). III. Title. IV. Series: 


In formation 


circular (United States. Bu- 


reau of Mines) ; 9069. 










TN295.U4 [TN775] 


622s [669'. 7221 


85-600303 



CONTENTS 



Page 



Abstract , , 

Introduction , 

Crystal chemistry of the alkali aluminum halides , 

The alkali halides , 

Aluminum chloride , 

Aluminum fluoride , , 

Cryolite , Na 3 A1F 6 

Potassium cryolite , K 3 A1F 6 

Elpasolite , K 2 NaAlF 6 

Chiolite, Na 5 Al 3 F li+ 

KAIF^ and NaAlF^ 

NaAlCl^ and KALCl^ 

Summary of crystal chemistry 

Phase equilibria 

Melting points and transition temperatures 

Alkali halides 

Aluminum halides 

Alkali hexaf luoroaluminates 

Chiolite 

Alkali tetraf luoroaluminates 

Alkali tetrachloroaluminates 

Alkali halide systems 

The alkali haloaluminates 

Sodium halide-cryolite systems 

The alkali chloroaluminates 

Mixed alkali halides 

Equilibria in the system NaCl-KCl-AlCl 3 -NaF-KF-AlF 3 

Summary of phase equilibria data 

Phase relations in the system NaCl-KCl-AlCl 3 -NaF-KF-AlF 3 

Experimental procedures 

Solid solubility in the elpasolite phase 

Subsolidus equilibria 

Powder diffraction data for KAlF tt and K 3 A1F 6 

Conclusions 

References 

ILLUSTRATIONS 

1. The crystal structure of A1C1 3 , projected on the (010) plane 

2. One layer of A1C1 3 , projected on the (001) plane 

3. The structure of hexagonal A1F 3 , projected on the (0001) plane 

4. The structure of hexagonal A1F 3 , projected on the (1120) plane 

5. The structure of cryolite, Na 3 AlF 6 , projected on the (001) plane 

6. The structure of K-cryolite, K 3 A1F 6 , projected on the (001) plane 

7. An alternative structure for K-cryolite, assuming space group P4/mnc 

8. The structure of elpasolite, K 2 NaAlF 6 , assuming space group Pa3 

9. The structure of one layer of chiolite, Na 5 Al 3 F 1 i + , projected on the (001) 

plane 

10. The structure of chiolite, projected on the (100) plane 

11. The structures of NaAlF 4 and KAIF4 

12. The structure of NaAlCl 4 , projected on the (001) plane 



1 
2 
3 
3 
5 
6 
7 
9 

10 
12 
15 
16 
16 
17 
17 
17 
18 
18 
18 
18 
19 
19 
20 
20 
21 
21 
23 
27 
27 
28 
28 
29 
30 
32 
34 



5 

6 

7 

8 

9 

11 

11 

13 

14 
15 
16 
17 



ii 

ILLUSTRATIONS — Continued 

Page 

13. The system NaF-KF and the system NaCl-KCl 19 

14. The systems NaF-NaCl and NaF-KCl 20 

15. The system NaF-AlF 3 and the system NaCl-AlCl 3 20 

16. The system KF-A1F 3 and the system KCI-AICI3 21 

17. The system NaF-Na 3 AlF 6 and the system NaCl-Na 3 AlF 6 21 

18. The system NaCl-KCl-AlCl 3 22 

19. The reciprocal system NaCT-KCl-NaF-KF , redrawn from the data of 

reference 47 22 

20. The reciprocal system NaCl-KCl-NaF-KF , redrawn from the data of 

reference 25 22 

21. The system NaCl-KCl-Na 3 AlFg-K 3 AlFg , redrawn from the data of reference 8.. 23 

22. Liquidus surfaces of the faces of the compositional prism for the system 

NaCl-KCl-NaF-KF-Na 3 AlF 6 -K 3 AlF 6 , redrawn from the data of reference 36.... 24 

23. The NaF-KCl-K 2 NaAlF 6 section through the compositional prism 25 

24. The NaF-NaCl-K^NaAlFg section through the compositional prism 25 

25. The KF-NaCl-K 2 NaAlF 6 section through the compositional prism 25 

26. The KF-KCl-K 2 NaAlF 6 section through the compositional prism 25 

27 . The Na 3 A1F 6 -K 3 A1F 6 binary diagram 26 

28. The system NaF-KF-AlF 3 26 

29. The system NaCl-NaF-ALF 3 27 

30. The system NaCl-KCl-NaF 27 

31. X-ray lattice measurements for samples in the system Na 3 AlFg-K 3 AlF 6 28 

32. X-ray intensity ratio measurements for samples in the system Na 3 AlF 6 - 

K 3 A1F 6 29 

33. Surfaces of the compositional prism for the system NaCl-KCl-AlCl 3 -NaF-KF- 

A1F 3 , showing subsolidus compatibility relationships 29 

34. Subsolidus compatibility tetrahedra in the system NaCl-KCl-AlCl 3 -NaF-KF- 

A1F 6 31 

35. Subsolidus compatibility in the portion of the system NaCT-KCl-AlCl 3 -NaF- 

KF-A1F 3 corresponding to those in reference 36 31 

36. Subsolidus compatibility tetrahedra in the volume bounded by NaCl-KCl- 

AlF 3 -Na 3 AlF 6 -K 3 AlF 6 31 

37. Subsolidus compatibility tetrahedra in the volume bounded by NaCl-KCl- 

A1C1 3 -A1F 3 31 

38. Section through the compositional prism of the system NaCl-KCl-AlCl 3 -NaF- 

KF-A1F 3 at the chloride-fluoride molar ratio of 1:1 32 

TABLES 

1. Crystallographic data and sources of powder diffraction data for crystal- 

line phases in the system NaCl-KCl-AlCl 3 -NaF-KF-AlF 3 4 

2. Measured and calculated interionic distances in the alkali halides 5 

3. Interionic distances in the cryolite structure 8 

4. Interionic distances in K 3 A1F 6 10 

5. Interionic distances in elpasolite 12 

6. Interionic distances in chiolite 15 

7. Interionic distances in KA1F 4 16 

8. Interionic distances in NaAlF 4 16 

9. Interionic distances in NaAlCl 4 17 

10. Powder diffraction data for KALF4 33 



ALUMINUM FLUXING SALTS: A CRITICAL REVIEW OF THE CHEMISTRY 
AND STRUCTURES OF ALKALI ALUMINUM HALIDES 

By Charles A. Sorrell, 1 John G. Groetsch, Jr., 2 and D. M. Soboroff 3 



ABSTRACT 

This Bureau of Mines publication reviews the structural characteris- 
tics of crystalline phases and phase equilibria data for the system 
NaCl-KCl-AlCl3-NaF-KF-AlF3 , which encompasses a large number of molten 
salt fluxes currently used in aluminum recycling. Its purpose is to 
provide guidelines for research into the relationships between molten 
salt compositions and their physical properties, notably vapor pres- 
sures, densities, surface tensions, and viscosities, knowledge of which 
is essential to maximizing fluxing efficiencies and metal recovery and 
minimizing hazardous emissions and disposal problems. 

In addition, this report describes experimental determinations of sub- 
solidus compatibility relationships in the system, which is of the qua- 
ternary reciprocal type, containing 12 different stable 4-phase assem- 
blages in the solid state. The compatibility diagram serves to define 
the important compositional planes across which important changes in 
properties are likely to occur. 



^Geologist. 
^Chemical engineer. 
^Research supervisor. 
Avondale Research Center, Bureau of Mines, Avondale, MD . 



INTRODUCTION 



This publication combines a review of 
technical information from the literature 
with experimental data associated with 
remelting of scrap aluminum and recycling 
of dross. Bureau of Mines research in 
this area is not new; in 1916, Gillett 
( 16)4 published a lengthy summary of the 
problem and reviewed the technology 
available at that time. The problems en- 
countered in melting aluminum are conse- 
quences of its extreme reactivity, ex- 
ceeded only by those of the alkali and 
alkaline earth metals. Were it not for 
the fact that a thin, transparent alumi- 
num oxide film forms on the surface of 
the metal immediately on exposure to the 
atmosphere and serves as an effective 
barrier to further reaction, aluminum 
would find few practical applications. 

During melting of scrap aluminum, for- 
mation of an oxide layer on the surface 
of the molten metal is unavoidable unless 
melting is accomplished in a high vacuum 
or completely inert atmosphere, neither 
of which is a feasible method. Reactions 
with the furnace atmosphere form not only 
aluminum oxide, AI2O3 , but also aluminum 
nitride, A1N, aluminum carbide, Al^C^, 
and a range of oxide and oxynitride spi- 
nels. Fortunately, even though the den- 
sities of these phases are greater than 
that of molten aluminum, some remain 
on the surface of the melt and can be 
skimmed prior to removal of the melt from 
the furnace. This skim, or dross, can 
contain as much as 85 pet metallic alumi- 
num trapped as droplets; these droplets 
are coated with a thin but remarkably 
strong layer of oxides which prevents the 
droplets from coalescing and combining 
with the metal in the bath. Larger alu- 
minum producers sell the skim or dross to 
smaller, independent operators who recy- 
cle it to recover a portion of the metal 
content. 

There are other problems with melting 
aluminum scrap. Because the material, 
which may include beverage cans, borings 
and turnings, foil, etc., has a high 

Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of this report. 



specific surface area, it oxidizes read- 
ily, and the oxide coating prevents the 
metal from coalescing into a pool. In 
practice, therefore, the scrap is added 
to a molten metal heel in a charging well 
and is pushed beneath the surface as rap- 
idly as possible. It has become standard 
practice to use a cover of molten salt on 
the surface of the liquid metal in the 
charging well to serve the multiple pur- 
poses of protecting the aluminum from 
further oxidation, stripping the oxide 
film from the molten metal so the drop- 
lets can coalesce, and holding the solid 
particles in suspension so that a clean 
metal can be recovered. 

The mechanisms by which the salt "flux" 
strips the oxide from the metal and holds 
the solids in suspension are not well un- 
derstood. Sully (60) provided a reason- 
able explanation of the mechanism for 
suspension of solids based on the obser- 
vation that 90 wt pet NaCl-10 wt pet CaF 2 
molten salts loaded with 10 to 15 wt pet 
alumina of various types behaved as thix- 
otrophic fluids, through which the set- 
tling velocities of solids were nearly 
zero. It was thought for many years that 
the fluxing salt dissolved the aluminum 
oxide from the surface of the metal, but 
it now appears that this is not a major 
factor. Phillips showed that, though the 
solubility of aluminum oxide in molten 
cryolite, Na 3 AlFg , is appreciable (45), 
addition of NaCl to the melt decreases 
the solubility to very low levels (46). 
It has since been suggested (65-66) that 
the low interfacial tension between the 
salt and molten aluminum is the primary 
cause of the stripping. Because the salt 
wets the metal and the oxide particles 
and, in turn, the metal does not wet the 
oxide, the stripping action is very ef- 
fective. It is also believed that the 
presence of fluorides in the salt is ben- 
eficial in lowering the interfacial ten- 
sion, enhancing the stripping action. 
For this reason, fluxing salts commonly 
are made up of chlorides with a small 
percentage of fluoride, normally cryo- 
lite, Na3AlFg , or fluorspar, CaF2» 

In the most recent review of the chem- 
istry and properties of salt fluxes used 



in secondary aluminum production pro- 
cesses, Rao (49) listed 122 flux composi- 
tions from the literature and 36 patented 
compositions , nearly all of which are 
mixtures of halides. The most commonly 
used fluxes are mixtures of NaCl and KC1 
in approximately equal amounts with 3 to 
5 wt pet cryolite added. The chloride 
mixture has a melting point near the min- 
imum liquidus temperature of ~645° C, 
just below the melting point of aluminum. 
This fact is important in the recycling 
of drosses in rotary kilns because, dur- 
ing heating of the charge, the salt melts 
and coats the aluminum before the metal 
melts and thereby protects the metal from 
further oxidation. The fluoride is added 
to enhance the fluxing action, though 
some operators obtain acceptable results 
without the fluoride. Of the 158 compo- 
sitions listed by Rao, 49 are formulated 
from various mixtures of NaCl, KC1, 
AICI3 , NaF, KF, and A1F 3 or from mixtures 
of compounds of these halides; the major- 
ity of the other compositions listed con- 
tain one or more of those six halides 
with small percentages of other salts. 

Study of the literature indicates that 
no truly thorough investigation of any 
fluxing salt system has been done. In 
the case of the alkali aluminum halides , 
the system itself has not even been de- 
fined. Changes made in fluxing salts 



since 1916 have used trial and error as 
bases , and all the salt properties and 
relevant reactions have not been deter- 
mined. There are abundant data on liqui- 
dus temperatures, vapor pressures, densi- 
ties, surface tensions, viscosities, and 
reactions with ambient atmospheres for 
small compositional ranges within many 
systems, but there is no complete charac- 
terization of an entire system. In view 
of the fact that incongruent vaporiza- 
tion, reactions with the metal and the 
suspended solids, and hydrolysis reac- 
tions with combustion products all are 
likely to occur, it is reasonable to ex- 
pect that molten salt compositions can 
vary rapidly with time over broad ranges , 
so it is essential that the whole chemi- 
cal system be defined and characterized. 
This information is also necessary to 
facilitate development of processes for 
recycling or safe disposal of spent salt 
slags (34) . Because of the wide use of 
compositions within the system NaCl-KCl- 
AlCl 3 -NaF-KF-AlF3 in present-day aluminum 
recycling, it has been selected as the 
first to be studied. 

This report is restricted to a discus- 
sion of structures and melting character- 
istics, obtained from the literature, and 
determination of compatibility relation- 
ships based on Bureau of Mines experimen- 
tal research. 






CRYSTAL CHEMISTRY OF THE ALKALI ALUMINUM HALIDES 



Because several researchers have shown 
relationships between melting and vapori- 
zation characteristics and structure of 
the crystalline solids, and because some 
of the structural characteristics of the 
solids have been observed in the molten 
state, data on the crystalline phases re- 
ported to be stable in the system NaCl- 
KCl-AlCl 3 -NaF-KF-AlF 3 are assembled here. 
Crystallographic data for the 14 phases 
in the system with references for the 
structural descriptions are listed in 
table 1. 

THE ALKALI HALIDES 

The alkali halides — NaCl (halite, rock 
salt), KC1 (sylvite), NaF (villiaumite) , 
and KF — are isostructural, crystallizing 



in the familiar rock salt structure, 
space group Fm3m (No. 225), with ions lo- 
cated in the following positions: 



11. 



Alkali : (4a) 000; -^0 



22 



V-; 
2 2' 



22 



Halide 



111 



(4b) - ^-; ^00; 0^0; 00^ 
222 2 2 2 



The alkali ions are coordinated with six 
halide ions on the corners of regular oc- 
tahedra; conversely, the halide ions are 
coordinated with six alkali ions (6:6 
coordination) so that each octahedron 
shares all eight faces with adjacent oc- 
tahedra. Interionic distances, calcu- 
lated from the lattice parameters in ta- 
ble 1, are listed in table 2. Comparison 



TABLE 1. - Crystallographic data and sources of powder diffraction data 
for crystalline phases in the system NaCl-KCl-AlCl 3 -NaF-KF-AlF 3 



Crystalline 
phase 


System and space group 


Cell 
parameters 1 


Calculated 
density, 
g/cm 3 


References 


PDF card 2 


NaCl 


Cubic, Fm3m 


a = 
a = 
a = 
a = 

a = 
b = 
c = 

3 = 


5.6402 
6.2431 
4.6342 
5.347 

5.92, 
10.22, 
6.16, 
108°. 


2.163 
1.987 
2.802 
2.524 

2.498 


3, 

3, 
48 
48 

28, 


61 

63 
, 63 
, 63 

62 




5-628 


KC1 




4-587 


NaF 
KF 




4-793 
4-726 


AICI3 


» 


22-10 


AIF3 




a = 
a = 


5.039, 
58.5°. 


3.197 


27, 


54, 


58 


9-138 






a = 
c = 


4.927, 
12.445. 


3.197 


27, 


54, 


58 


NAp 


Na 3 AlF 6 




a = 
b = 
c = 

3 = 


5.46, 

5.61, 

7.80, 

90.18°. 


2.918 


35, 


43, 


59 


25-772 


K 3 A1F 6 


Tetragonal, 14/mmm or 


a = 
c = 


5.944, 
8.468. 


2.867 


6, 


20, 


22, 59 


3-615 


K 2 NaAlF 6 


Cubic, Pa3 or Fm3m. . . . 


a = 


8.1120 


3.013 


21, 


38, 


59, 62 


22-1235 


Na 5 Al 3 F 11+ 


Tetragonal, P4/mnc. . . . 


a = 
c = 


7.0142, 
10.400. 


2.997 


4, 


42 




30-1144 


KAIF4 


Tetragonal, P4/mmm. . . . 


a = 
c = 


3.550, 
6.139. 


3.049 


5, 


44 




2-595 


NaAlF^ 




a = 
c = 


3.48, 
6.29. 


2.746 


15, 


17, 


24, 37 


19-1243 


NaAlCl,, 


Orthorhombic, P2 1 2 1 2 1 . 


a = 
b = 
c = 


10.36, 
9.92, 
6.21. 


1.996 


1, 


53 




23-649 


KAICI^ 




a = 
b = 
c = 

3 = 


7.23, 
10.48, 

9.25, 
93.3°. 


1.973 


53 






23-468 



Nap Not applicable. 

! Unit cell parameters reported in angstroms at room temperature (20°-26° C). 

2 Powder Diffraction Files, compiled by the Joint Committee on Powder Diffraction 
Standards, International Centre for Diffraction Data, 1601 Park Lane, Swarthmore, PA 
19081. 




FIGURE 1. - The crystal structure of AIC 1 3/ projected on the (010) plane. Small atoms are Al; large atoms 
are CI. 



TABLE 2. - Measured and calculated interionic distances 
in the alkali halides, MX 



Crystal 


Measured 


Calculated 1 


Crystal 


Measured 


Calculated 1 




M-X 


X-X 


M-X 


X-X 


M-X 


X-X 


M-X 


X-X 


NaCl 


2.802 
3.147 


3.988 

4.450 


2.76 
3.24 


3.62 
3.62 


NaF 


2.367 
2.674 


3.277 
3.781 


2.31 
2.69 


2.72 






2.72 



Calculations were made using the following radii: 
1.81 A; F", 1.36 



of measured and calculated interionic 
distances shows a rather stable configu- 
ration, with the alkali-halide values 
near the potential minimum and little 
anion-anion repulsion. The alkali ha- 
lides are, therefore, very stable phases, 
as will be shown later. 



Na + , 0.95 A; K + , 1.33 A; CI", 



ALUMINIUM CHLORIDE 



The layer structure of aluminum chlo- 
ride, AICI3, is shown in figures 1 and 2. 
The structure is monoclinic, space group 
C2/m (No. 12), with atoms in the follow- 
ing positions: 



Al : (4g) ± (oyO; y'y + y,0^) ; y=0.167 



Cl(l) : (4i) ± 



xOz; y+x.p 



x=0.226; z=0.219 



Cl(2) : (8j) 



xyz; xyz; y+x,y+y,z; 



1 1 
Y + x,j-y,z 



x=0.250; y=0.175; z=0.781 



The structure consists of a distorted 
close-packed arrangement of chloride ions 
in which all the octahedral sites are 
empty in one layer and two-thirds of the 
octahedral sites are occupied by alumi- 
num ions in adjacent layers. The layers 
are held together by weak van der Waals 
bonds. Within the bonded layers, the oc- 
tahedra share all six corners with adja- 
cent octahedra. The AlClg octahedra are 
all identical, slightly distorted, with 
two Cl~ ions at 2.29 A, two at 2.32 A, 
and two at 2.33 A from the Al 3+ ion. The 
octahedral edges are much shorter than 
the sum of the anionic radii, 3.62 A, 
with two Cl-Cl distances of 3.10 A, two 
of 3.28 A, and eight of 3.33 A. This 
is because the radius ratio, 0.28, is 



too small for a stable octahedral coor- 
dination. A structure with tetrahedral 
coordination is not possible, however, 
because of the requirements of the elec- 
trostatic valence rule. The structure 
is, therefore, a compromise between con- 
flicting physical requirements; the high 
vapor pressures and low melting point, as 
discussed later, are a consequence of the 
unstable structure. Aluminum chloride is 
essentially isostructural with gibbsite, 
A1(0H) 3 . 

ALUMINUM FLUORIDE 

Unlike the layer structure of A1C1 3 , 
the structure of aluminum fluoride, 
AIF3 , is a continuous, three-dimensional 




FIGURE 2. - One layer of AICI3, projected on the (001) plane, showing pseudohexagonal symmetry. 



framework of AlFg octahedra, with shared 
corners, and is much more stable. It is 
rhombohedral, space group R32 (No. 155) , 
with atoms in the following positions: 

Al : (2c) xxx; xxx; x=0.237 

F(l) : (3d) Oxx; xOx; xxO; x=0.430 



F(2) : (3e) ^xx; x|x; xx^; x=0.070 



The corresponding hexagonal coordinates 
are — 

Al : (6c) OOz; OOz; Rh; z=0.237 

F(l) : (9d) xOO; 0x0; xxO; Rh; x=0.430 



F(2) : (9e) xo|; 0x-|; xx|; Rh; x=0.570 



The structure, as shown in figures 3 and 
4, is a deformed version of the simple 
cubic Re0 3 structure, with a slight 



expansion along one [111] axis. It is a 
defect cubic close-packed arrangement of 
F~ ions, with one-fourth of the sites va- 
cant. All the available octahedral sites 
are occupied by Al 3+ ions. The octahedra 
share all six corners with adjacent octa- 
hedra, forming a three-dimensional frame- 
work structure. The octahedra are dis- 
torted, with three F" ions at 1.707 A and 
three at 1.889 A from the central Al 3+ 
ion. The 12 edges of the octahedra are 
formed by F~ ions 2.537 A apart. The F-F 
distances are somewhat less than the sum 
of the radii, 2.72 A, and are most likely 
an indication of considerable polari- 
zation of the F~ ion. The stability 
of A1F 3 , as compared with AICI3, is ob- 
viously a consequence of the stable 
octahedral coordination and the three- 
dimensional linkage. 

CRYOLITE, Na 3 ALF 6 

Cryolite, Na 3 AlF 6 , is monoclinic, space 
group P2/n (No. 14); this places the unit 




FIGURE 3. - The structure of hexagonal AIF 3 , projected on the (0001) plane. Small atoms are Al, large atoms 
are F. 




FIGURE 4. - The structure of hexagonal AIF 3 , pro- 
jected on the (1120) plane. 

cell in an alternate setting of space 
group P2 1 /c. The atoms are located at — 



Al : (2a) 000; 



111 
222 



Na(l) : (2b) Oo|; —0 



Na(2) : (4c) ± 



xyz; -+ x ,--y,-+z 



x=0.50; y=0.945; z=0.24 

F(l) : (4c) x=0.065; y=0.06; z=0.22 

F(2) : (4c) x=0.71; y=0.16; z=0.03 

F(3) : (4c) x-0.15; y=0.28; z=0.94 

The structure, shown in figure 5, is a 
pseudocubic close-packed arrangement of 



F" ions with one-fourth of the sites va- 
cant. The octahedral sites are occupied 
by Al 3+ and one-third of the Na + ions in 
an ordered, alternating arrangement, 
forming a framework structure with all 
octahedra sharing corners with adjacent 
octahedra. The octahedral framework of 
the cryolite structure is a distortion of 
the AIF3 structure, with half the sites 
occupied by Na + ions, rather than Al 3+ 
ions. The remaining Na + are in inter- 
stitial sites with a highly distorted 
octahedral coordination. Interionic dis- 
tances in the cryolite structure are 
listed in table 3. The Al-F and Na-F 
distances are comparable to those in AIF3 
and NaF , respectively, and the F-F dis- 
tances in the AlFg octahedron are slight- 
ly less than the sum of the F~ radii, in- 
dicating a strongly bonded structure. As 
is indicated later, the high-temperature 
stability of cryolite is comparable with 
those of NaF and A1F 3 . 

TABLE 3. - Interionic distances in the 
cryolite structure 



Ions 


Number 

of 

ions 


Interionic 
distance, 
A 




2 

4 


1.783 
1.834 




2 
4 


2.332 
2.237 


Na-F , interstitial. . . 


1 
2 
2 
1 


2.211 
2.338 
2.466 
2.778 


F-F, A1F 6 octahedron. 


2 
2 
2 
4 
2 


2.500 
2.552 
2.562 
2.594 
2.617 


F-F, NaF octahedron.. 


2 
2 
2 
2 
2 
2 


2.985 
2.988 
3.206 
3.242 
3.333 
3.449 




FIGURE 5. - The structure of cryolite, Na3AlF6, projected on the (001) plane. Small atoms are Al, intermedi- 
ate-sized atoms are Na, large atoms are F. 



POTASSIUM CRYOLITE, K 3 A1F 6 

The exact structure of I^AlFg , commonly 
referred to as K-cryolite, has not been 
determined. It was originally described 
as cubic (6) and is still indexed as 
such in the incomplete data in the Powder 
Diffraction Files. (See table 1.) Later 



work showed, however, that it is tetra- 
gonal, with parameters near those listed 
in table 1. If it is assumed that the 
structure is a tetragonal distortion of 
the structure originally assigned to it, 
the correct space group is 14/mmm (No. 
139) , with atoms located at — 



10 



Al 



(2a) 000; 



111 
222 



K(l) : (2d) 00^; || 



1«3 



K(2) : (4d) Oil; |oi; ^ 



13_ 
24 



F(l) : (4e) ± (oOz; 1,I,I +Z ) ; z=0. 



20 



F(2) : (8h) 



±r 



xxO; xxO; y+x.j 



1 

hx 'T ; 



i i 

X, X,— 

*2 *2 



x-0.20 



The structure, based on these data, is 
shown in figure 6. Though the lattice 
parameters and symmetry are different 
from those of cryolite, Na3AlF 6 , the 
structure of Na 3 AlF 6 is quite similar. 
It consists of a framework of alternating 
AlFg and KF 6 octahedra, with the remain- 
ing K + ions in interstitial sites. Un- 
like the Na + ion, which occupies the 
highly distorted interstitial octahedral 
site, the larger K + props the structure 
open and occupies a distorted cubo- 
octahedral position. The interionic dis- 
tances are listed in table 4. The Al-F 
distances are significantly less than in 
cryolite (table 3), the K-F distances are 
less than in KF (table 2), and the F-F 
distances in the AlFg octahedra are 
greater than in cryolite (table 3). This 
indicates the structural data are not 
very accurate. The correct space group 
is probably P4/mnc (No. 128), and the 
structure is a slight distortion of that 
shown in figure 6. This alternative 
structure is shown in figure 7. The dif- 
ferences between the structures are very 
slight and not significant in terms of 
the stability of the phase; it is to be 
expected that high-temperature properties 
of l^AlFg will be comparable to those of 
Na 3 AlF 6 , as proves to be the case. 



TABLE 4. - Interionic distances 
in K 3 A1F 6 





Number 


Interionic 


Ions 


of 


distance, 




ions 


A 




4 


1.681 




2 


1.694 




4 


2.540 




2 


2.522 


K-F , interstitial. . . . 


4 


3.002 




8 


3.012 


F-F, A1F 6 octahedron. 


4 


2.378 




8 


2.386 


F-F, KF 6 octahedron.. 


4 


3.566 




8 


3.580 



ELPASOLITE, K 2 NaAlF 6 

The structure of elpasolite, as orig- 
inally described (38), was placed in 
space group Pa3 (No. 205), with atoms 
at — 



Al: (4a) 000; ±0^s ±|o ; o|L 



11 



Na: (4b) ill; 0^0; 0o|; ^00 



K: (8c) ± fxxx, -+x,-i-x,x; x,-j+x,-|-x; I- x ,x,-UxJ ; x=0.25 
F: (24d) ± f xyz; zxy; yzx; ^-+x,--y,z; -+z,--x,y; 



11 _ _ 1 1 _ 1 1 

-^ + y,-j-z,x; x,-+y,--z; z,-+x,--y; 



_ 1 1 1 _ 1 1 _1 1 _ 1 \ 

y, "2 'T x; "T x »y»7 +Z ' ^- z » x »7 + y; Y y ' z >~2 ) ; 

x=0.22; y=0.03; z=0.01 




FIGURE 6. - The structure of K-cryolite, K 3 AIF 6 , 
projected on the (001) plane. Space group is assumed 
tobe 14/mmm. Small atoms are Al; intermediate-sized 
atoms are K; large atoms are F. 




'80 ><*^ so w 

FIGURE 7. - An alternative structure for K-cryolite, 
assuming space group P4 mnc. 



12 



The structure thus defined is shown in 
figure 8. As is the case with cryolite, 
the structure consists of a framework 
of alternating A1F 5 and NaF 6 octahedra 
linked by shared corners in all three 
dimensions. The K + ions are all located 
in interstitial cubo-octahedral sites, as 
is the case with K^AlFg. The higher sym- 
metry is a consequence of the ordering 
of Na + and K + ions in octahedral and 
cubo-octahedral sites, respectively. The 
similarity between the elpasolite struc- 
ture and the structures of Na3AlFg and 
K 3 A1F 6 (figs. 5-7) may be seen in the 
tetragonal subcell indicated by dot-dash 
lines in figure 8. The interionic dis- 
tances are listed in table 5. The metal- 
fluoride distances are comparable to 
those in cryolite and those in A1F 3 , NaF, 
and KF , so it is reasonable to expect the 
high-temperature properties to be compar- 
able with those phases, as proves to be 
the case. 

There is some question in the litera- 
ture about the details of the structure 
of elpasolite. Though Menzer (38) placed 
the structure in space group Pa3, several 
other authors (21 , 59 , 62 ) contend that 
X-ray and paramagnetic resonance data 
show the correct space group to be Fm3m. 
The atomic coordinates listed above show 
that the differences between the two 



Al (1): (2a) 000; 



111 
222 



Al (2): 

Na ( 1 ) : 

Na (2): 

F (1): 

F (2): 



(4c) 0^0; ^00; 0^-; i-O^ 

2 ' 2 ' 22' 2 2 



(2b) 00J; ||0 



TABLE 5. - Interionic distances 
in elpasolite 





Number 


Interionic 


Ions 


of 


distance, 




ions 


A 




6 


1.805 




6 


2.289 


K-F, interstitial.... 


3 


2.656 




3 


2.777 




3 


3.005 




3 


3.113 


F-F, AlFg octahedron. 


6 


2.306 




6 


2.778 


F-F, NaFg octahedron. 


6 


3.127 




6 


3.343 



choices are very slight. If the x, y, 
and z coordinates of the fluoride ion 
were shifted from 0.22, 0.03, and 0.01 
to 0.25, 0.00, and 0.00, respectively, 
the symmetry would be changed from 
Pa3 to Fm3m, so the differences are 
insignificant. 

CHIOLITE, Na 5 Al 3 F 11+ 

Chiolite is tetragonal, space group 
P4/mnc (No. 128), with atoms located at — 



1 . 11 



1 1 



(8g) ± I x,^+x,-^; ^+x,x,^; ^- x ,x,f-; x,^+x,f ); x=0.275 



1 . 

"'2 'h 



(4e) ± (oOz; 1,1,1+z^ ; z =0.185 

(8h) ± (xyO; -U x i-x,j; yxO ; I + y,i +x I); x=0.07; y=0.25 



13 



xyz; xyz; I +x ,i-y,I + z; ~x,I + y i + z; 



yxz; yxz; -|+y,-|+x; -Uz ; I-y,I- x ,i+z 



x=0.21; y=0.535; z=0.12 



a. 




FIGURE 8. - The structure of elpasolite, K 2 NaAIF6, assuming space group Pa3. In order of increasing size, 
the atoms are Al, Na, F, and K. 



14 



One of the complex layers making up 
the structure is shown in figure 9. The 
layer is composed of two kinds of A1F 6 
octahedra; half the octahedra share four 
corners, and the other half share two 
corners with adjacent octahedra. One- 
third of the Na + ions are located within 



the layer, surrounded by eight equidis- 
tant F~ ions. The remaining Na + ions are 
located between the layers, as shown in 
figure 10, surrounded by F~ ions in a 
highly distorted octahedral arrangement. 
There is no octahedral linkage between 
layers; the Na-F bonds serve to hold the 




FIGURE 9. - The structure of one layer of chiolite, Na 5 AI 3 F 14 , projected on the (001) plant 



15 



0|0-0 




(g£) 









-$© — 



75 



(© 



25 



93 




07 



75 



25 



FIGURE 10. - The structure of chiolite, projected 
on the (100) plane. 

layers together. The interionic dis- 
tances, listed in table 6, are comparable 
to those in A1F 3 , cryolite, and NaF, in- 
dicating stabilities comparable with 
those phases. 

KA^ AND NaAlF^ 

The potassium and sodium tetrafluoro- 
aluminates are isostructural, space group 
P4/mmm (No. 123), with atoms located at — 

Na or K: (la) 000 

111 



Al: (Id) 



222 



F(l): (2e) 0?±; jo| 



F(2): (2h) 



11 11_ n „. 
-— z; — -z; z=0.21 
22 * 22 ' 



TABLE 6. - Interionic distances 
in chiolite 



Ions 


Number 
of 


Interionic 
distance, 




ions 


A 


Al(l)-F, octahedron.. 


6 


1.821 


Al(2)-F, octahedron.. 


2 
4 


1.821 
1.946 


Na(l)-F, intralayer. . 


8 


2.399 


Na(2)-F, interlayer. . 
F-F, Al(l)-F 


2 
2 

1 

1 

12 

2 
4 
4 
2 


2.207 
2.273 
2.582 
2.611 

2.575 


F-F, Al(2)-F 


2.496 




2.553 
2.773 
2.987 



The structure, shown in figure 11, con- 
sists of layers of AlFg octahedra, each 
sharing four corners with adjacent octa- 
hedra. All the Na + or K + ions are lo- 
cated between the layers, surrounded by 
eight F~ ions on the corners of a tetra- 
gonal prism. The interionic distances 
are listed in tables 7 and 8. The Al-F 
and F-F distances are comparable with 
those in A1F 3 and cryolite, but the Na-F 
and K-F distances are significantly 
greater than in the corresponding alkali 
fluorides and hexaf luorides (tables 2-5). 
The K-F distance in KAlF^ is 5.5 pet 
greater than in KF , leading to weak bond- 
ing between the layers, accounting for 
the low melting point (5, 44). The Na-F 
distance in NaAlF^ is 18 pet greater than 
in NaF; this is in accord with the obser- 
vation that NaAlF^ is unstable with re- 
spect to chiolite, Na 5 Al 3 F 14 , and has 
probably been formed only as a metastable 
phase via vapor deposition (5, 24, 37), 
although it has been reported as a stable 
phase (17). 



16 




TABLE 7. - Interionic distances 
in KAIF^ 



FIGURE 11. - The structures of NaAIF 4 and KAIF 4 . 
Alkali ions are shown as intermediate-sized circles; 
small atoms are Al; large atoms are F. 

NaAlCl^ AND KA1C1 4 

Ther structure of NaAlCl^ has been as- 
signed to the orthorhombic space group 
P2i2i2i (No. 18), with atoms located in 
the general positions — 

(4a): xyz; --x,y,z; -+x,--y,2; x,-+y,--z 

with the following coordinates: 



Atom 


X 


y '* 


X 


Na 


0.128 


0.207 


0.677 


Al 


.039 


.485 


.204 


Cl(l) 


.031 


.490 


.552 


Cl(2) 


.148 


.316 


.105 


Cl(3) 


.348 


.024 


.923 


CK4) 


.379 


.336 


.577 



The structure, shown in figure 12, is 
made up of independent AlCl^ tetrahedra 
held together by Na + ions in the inter- 
stitial positions. The interionic dis- 
tances are listed in table 9. The Al-Cl 
distances are somewhat less than the 



Ions 


Number 

of 

ions 


Interionic 
distance, 

A 


Al-F 


4 
2 

8 

8 

4 

1 


1.775 


K-F 


1.783 
2.822 




2.510 
2.514 

2.578 



TABLE 8. - Interionic distances 
in NaAlF^ 



Ions 


Number 

of 

ions 


Interionic 
distance, 
A 


Al-F. 




4 

2 

8 

8 
4 

1 


1.740 


Na-F, 




1.824 
2.793 


F-F, 
F-F, 




2.461 
2.521 

2.642 



octahedral distances in AICI3, indicating 
a more stable coordination. Only two of 
the Na-Cl distances, however, are near 
the value of 2.802 A in the NaCl struc- 
ture (table 2), explaining the low melt- 
ing point for the phase. 

X-ray diffraction analysis of the 
structure of KAICI^ has not been done. 
The unit cell is monoclinic, rather than 
orthorhombic, but Raman spectroscopy (53) 
indicates an independent tetrahedral 
structure, probably a distortion of the 
NaAlCl^ structure. 

SUMMARY OF CRYSTAL CHEMISTRY 

The crystal structures of phases in the 
system NaCl-KCl-AlCl 3 -NaF-KF-AlF 3 are es- 
sentially of two groups: (1) those in 
which all the metal-halide distances are 
such that the interionic potential energy 
is near the minimum and the linkage of 



17 



®^e . 0... 0^0 J£ 0^0 



TABLE 9. - Interionic distances 



in NaAlCl, 



Ions 1 



Interionic 
distance, 
A 



T03 © ©T03 © ©T© 



Al-Cl, tetrahedron. 




FIGURE 12. - The structure of NaAICI 4 , projected 
on the (001) plane. Small atoms are Al; intermediate- 
sized atoms are Na; large atoms are F. AIF4 tetra- 
hedra are shown in the lower unit cells. 



Na-Cl, interstitial. 



Cl-Cl, tetrahedral edges, 



2.113 
2.121 
2.132 
2.163 

2.793 
2.877 
2.964 
3.054 
3.081 
3.290 

3.388 
3.468 
3.486 
3.494 
3.509 
3.543 



*1 ion in all instances. 



octahedral units, through shared faces 
or corners, produces a three-dimensional 
network, and (2) those in which geomet- 
ric and electrostatic conditions are 
such that neither of those two conditions 
can be met, so that some ions are in 
sites too large or the development of a 
three-dimensional network is not pos- 
sible. The former group, consisting of 
the alkali halides, A1F 3 , cryolite, 
K-cryolite, elpasolite, and chiolite, 
have relatively high melting points and 
low vapor pressures. The latter group, 



consisting of AICI3 and the alkali 
tetraf luoroaluminates and tetrachloro- 
aluminates , have relatively low melting 
points and high vapor pressures. The re- 
lationship between crystalline structure 
and the structure and properties of the 
liquid phase is not clear, but it is rea- 
sonable to expect a dependence on the ra- 
tios of the different ions, particularly 
as those ratios will affect the distribu- 
tion of aluminum halide octahedral and 
tetrahedral species in the raelt. 



PHASE EQUILIBRIA 



A large body of literature describing 
the melting characteristics of composi- 
tions within the system NaCl-KCl-AlCl 3 - 
NaF-KF-AlF 3 exists, but it is by no means 
complete, nor is it entirely reliable. 
There are numerous disagreements as to 
melting temperatures and many contradic- 
tions as to the compositions of the crys- 
talline phases. Following is a summary 
of the most important information now 
available. 



MELTING POINTS AND TRANSITION 
TEMPERATURES 

Alkali Halides 



The chlorides and fluorides of sodium 
and potassium are remarkably similar, not 
only in structure, but also in terras of 
thermal properties. The melting points 
reported are 800° C (30) to 805° C (_U) 
for NaCl; 722° C (47) to 774° C (11) for 



18 



KC1; 990° C (47) to 994.5° C (23) for 
NaF; and 850° C (47) to 858.4° C (23) for 
KF. Equilibrium vapor pressures for 
these phases are relatively low, and they 
show little tendency to react with atmos- 
pheric moisture to hydrolyze to the oxide 
and the acid. The high melting points 
and low vapor pressures are consequences 
of the intrinsic stability of the rock 
salt structure; the ions are in stable 
coordination and the polyhedral sharing 
results in a strongly bonded, isotropic 
structure. 

Aluminum Halides 

The aluminum halides are strikingly 
different in structure and properties. 
The unbonded layer structure (figs. 1-2) 
and the unstable octahedral coordination 
of AICI3 result in a very unstable phase. 
Melting points of 190.2° C (26) and 
193.3° C (30^) have been reported; these 
values were obtained from visual observa- 
tion of crystallization in sealed glass 
tubes at pressures greater than 760 torr. 
At atmospheric pressure, AICI3 sublimes 
at 180.2° C, and the triple point for co- 
existence of solid, liquid, and vapor has 
been placed at 192.6° C and 1,715 torr 
(_56 ) . In the presence of atmospheric 
moisture, AICI3 hydrolyzes readily to 
form A1(0H) 3 and HC1 , so it is necessary 
to conduct experimental work in a com- 
pletely dry environment. 

The three-dimensional octahedral frame- 
work and stable coordination of AIF3 
result in a much more stable phase. The 
melting point has been reported as 990° C 
in a sealed container ( 44 ) , but this 
value cannot be correct because the phase 
sublimes at 1,265° C at 760 torr ( 50 , 
64) ; the triple point has not yet been 
determined. AIF3 also hydrolyzes read- 
ily in the presence of atmospheric moi- 
ture to form A1(0H) 3 or A1 2 3 , depending 
on temperature, and HF. 

Alkali Hexaf luoroaluminates 

Cryolite, Na 3 AlF 6 , undergoes a reversi- 
ble transition from monoclinic to cubic 
symmetry at 561° C (35); melting tempera- 
tures between 1,000° C (36) and 1,009° C 
( 10) have been reported. The relatively 



high melting point and low vapor pres- 
sures are consequences of the three- 
dimensional octahedral network formed by 
alternating AlFg and NaFg octahedra. 
K-cryolite, K 3 A1F 6 , which is structurally 
very similar to cryolite, also has a rel- 
atively high melting point, 985° C (44) 
to 986° C (36). Steward ( 59 ) reported 
K 3 A1F 6 to be tetragonal, pseudocubic at 
25° C, with the c/a ratio changing gradu- 
ally to 300° C, at which temperature it 
is strictly cubic. 

Elpasolite, which has been given the 
formula K 2 NaAlF 6 , has not been well 
studied. Only one value of the melting 
point, 932° C, has been reported (36), 
and no property measurements have been 
made. As indicated later in this report, 
the phase has a wide range of composi- 
tion and is strictly cubic. It is to be 
expected, because of the chemical and 
structural similarity between elpasolite, 
cryolite, and K-cryolite, that the prop- 
erties will be similar across the whole 
system Na 3 AlF 6 -K 3 AlF 6 . 

Chiolite 

Chiolite, Na 5 Al3F 1 i + , melts incongruent- 
ly at 741° C to cryolite and liquid con- 
taining 30 wt pet AIF3 (13). The rela- 
tively high melting point is a result of 
the stable coordination of the Na + ion 
between the octahedral layers, but the 
structure is considerably less stable 
than that of cryolite. 

Alkali Tetrafluoroaluminates 

The phase KAlF^ has definitely been es- 
tablished as a stable phase in the sys- 
tem KF-AIF3. It melts incongruently at 
574° C to form AIF3 and a liquid only 
slightly richer in KF (44). The symmetry 



KA1F, 



as a function of temperature 



of 

is the subject of some disagreement. 
Grjotheim (17) reported the phase to be 
tetragonal at 25° C, with a transition to 
cubic symmetry at 327° C; Phillips (44) 
later reported it to be cubic at 25° C, 
with a transition to orthorhombic symme- 
try at approximately -15° C. 

The sodium analog, NaAlF^, which is 
isostructural with KAIF4, is much less 
stable, and its existence as a stable 



19 



phase at atmospheric pressure has been 
questioned. Grjotheim (17) , for example, 
determined a melting point of 775° C, 
using thermal analyses. Foster (13), 
however, did a detailed study which indi- 
cated it did not form as a stable phase. 
Other work (1_0, 15, 24, 37, _39) indicates 
that it forms only as a metastable phase, 
most readily by vapor deposition, and 
is easily dissociated into Na 5 Al 3 F 1 i + and 
A1F 3 . The layer structure of KA1F 4 and 
NaAlFt^ is such that, though the K + ion is 
large enough to maintain a stable struc- 
ture, the Na + ion is not able to achieve 
a stable Na-F bond distance. 

Alkali Tetrachloroaluminates 

As indicated by the structural analy- 
sis (1_) , the Na-Cl bond distances are 
considerably greater than the sum of the 
ionic radii; the resultant instability 
of the phase is shown by the low melt- 
ing point and high vapor pressures. 
The phase melts incongruently at 152° C 
(26) or 153° C (30) to NaCl and a liquid 
only slightly richer in A1C1 3 . The 
KAICI^ phase has a higher melting point, 
250° C ( 26 , 55) , also incongruent, form- 
ing KC1 and a liquid only slightly richer 
in AICI3 . The higher melting point of 
KAICI^ is a result of the larger radius 
of the K + ion, which is more stably coor- 
dinated with the CI" ions. 

ALKALI HALIDE SYSTEMS 5 

The systems NaCl-KCl ( 11 , 52 ) and NaF- 
KF (22), shown in figure 13, illustrate 
clearly the chemical differences between 
fluoride and chloride systems. NaCl and 
KC1 form a complete solution series with 
a minimum liquidus temperature of 645° C 
at ~50 mol pet KCl; the solid solutions 
dissociate spinodally at lower tempera- 
tures, however. The consolute point is 
at ~500° C and ~35 mol pet KCl. Dissoci- 
ation of the solid solutions proceeds at 
a rapid rate, with metastable equilibrium 
attained within an hour even at 25° C. 

5 These and other diagrams in this re- 
port have been drawn from data of the 
original authors or redrawn from those in 
"Phase Diagrams for Ceramists" (31-33). 



1,000 


1 r - 

_NaF 


1 1 i 1 r 


1 1 


900 


NaF and liquid 




KF 


800 


NaCl 


^\72i;^^^^ 


KF and liquid^^-^^ 


700 






^^^-^-^ 1 






-^-^\~64£__^- : ^^^-- 


\ - 


600 


(Na.K)CI solid solutions 


500 






- 


400 






\. 


300 




2 solid solutions 


N. - 


200 








100 


1 1 


1 1 1 1 1 


1 1 



NaF 
NaCl 



40 50 6C 

HALIDE. mol pet 



KF 
KCl 



FIGURE 13. - The system NaF-KF, drawn from the 
data of reference 23, and the system NaCl-KCl, drawn 
from the data of references 11 and 52. 

NaF and KF exhibit only slight solid so- 
lution on the KF-rich side, 5 mol pet NaF 
in KF; a eutectic is located at 721° C, 
60 mol pet KF. The differences may be 
attributed to the fact that the F~ ion 
has a strong polarizing effect on the ca- 
tions, making the structure much less 
adaptable to substitutional solid solu- 
tion. The liquidus minimum, 645° C, in 
the system NaCl-KCl is just below the 
melting point of pure aluminum, 660° C, 
so a 50:50 mix of the two salts makes an 
ideal base composition for fluxing salts. 

Interactions between NaF and the alkali 
chlorides, NaCl (18) and KCl (47), are 
very similar, as shown in figure 14. 
Both are simple eutectic systems, with 
no solid solubility; the eutectics and 
liquidus curves are quite close. The ab- 
sence of substitutional solid solution 
between CI" and F~ is another indication 
of their intrinsic differences and the 
extent to which the F~ ion determines the 
properties of molten salts. 

The remaining combination of mixed 
salts, NaCl-KF , is an unstable system; 
the phases react completely and irrevers- 
ibly to form NaF and KCl (47). 



20 



1,100 


I 


T 1 


— i 1 r 


1 i 


1 


yxx 










- 


900 










- 


800 


NaF 


liquid 






NaCI 


700 








; ^ 68 S>6^^' 


- 




~\64B>^ 










73% 




600 












500 










" 


400 






2 solids 




- 


300 










■ 


200 


- 








- 


100 


1 


1 1 


1 1 


1 1 


- 



30 40 50 60 

CHLORIDE, mol pel 



100 





NaCI 


NaF 


KCI 


NaCI 



FIGURE 14. - The systems NaF-NaCI and NaF-KCI, 
drawn from the data of references 18 and 47. 



THE ALKALI HALO ALUMI NATES 

The systems NaF-AlF 3 (_10, L3) and NaCl- 
A1C1 3 (30) , shown in figure 15, strik- 
ingly illustrate the fundamental differ- 
ence between the effects of AIF3 and 
ALCI3 on the properties of salt systems. 
The structural stability of cryolite, 
Na 3 AlF 6 , with a melting point higher than 
NaF, is the dominant factor in the system 
NaF-AlF 3 ; the eutectic is at 888° C, -13 
mol pet AIF3. Chiolite, Na 5 Al 3 F lt+ , melts 
incongruently to the more stable cryolite 
and a liquid with ~41 mol pet A1F 3 ; the 
eutectic between chiolite and A1F 3 is at 
694° C, -46 mol pet A1F 3 . The liquidus 
curve for A1F 3 has not been determined; 
the high vapor pressures over AlF 3 -rich 
liquids are such that sublimation of 
solid AIF3 in the assemblage probably oc- 
curs at a composition near the eutectic 
at atmospheric pressure. 

The unstable coordination of the Al 3+ 
ion with CI" in A1C1 3 and the unstable 
coordination of the Na + ion with CI" in 
the NaAlCl 4 structure result in a very 
unstable system in NaCl-AlCl 3 . The 




40 50 

HALIDE, mol pel 



FIGURE 15. - The system NaF-AIF3, drawn from 
the data of references lOand 13, and the system NaCI- 
AICI3, drawn from the data of reference 30. 

incongruent melting point of NaAlCl^ is 
152°-153° C, and the eutectic between 
NaAlCl^ and A1C1 3 is at 107.2° C, 61.4 
mol pet AICI3. The liquid miscibility 
gap, which has not been defined, extends 
from 191.3° C upward. 

The systems KF-AIF3 (44) and KC1-A1C1 3 
( 26 , 40 , 55) , shown in figure 16, are 
quite similar to the sodium analogs. 
There is no potassium analog of chiolite, 
however, and KAIF^ is a stable solid be- 
low 574° C. The greater stability of the 
K + ion in the structure of KAlCl^ results 
in a melting point of 250° C, as compared 
with 152°-153° C for NaAlCl^. 

SODIUM HALIDE-CRYOLITE SYSTEMS 

The NaF-rich portion (Na 3 AlF 6 ) of the 
system NaF-AlF 3 , shown previously in fig- 
ure 15, as reported by Fuseya (14) , and 
the system NaCl-Na 3 AlF 6 (29) are shown 
for comparison in figure 17. Addition of 
NaCI to cryolite has a greater effect on 
liquidus temperatures than does addition 
of NaF. This is possibly a result of the 
competition between the Al 3+ ion and the 



21 



,000 




I I 


I 


I I I 


1 1 1 


900 






K,AIF S \ 




S^ 


800 








/ 












AlF . and liquid 


700 










- 


600 


- 






x H 




_ 




KAIF, 








\ 


500 


— 




\ 


- 


400 




KCI and liquid 


\ 


- 


300 






\ 


/ \ 

/ 2 liquids ' 












?no 






KAICI. 

1 




1 J 




\^^^"~"aICI, liquid 


100 




I I I 


1 1 1 1 



KF 

KCI 



40 50 60 

HALIDE, mol pet 



100 
AlF, 
AICI, 



FIGURE 16.- The system KF-AIF3, drawn from the data 
of references 26 and 44, and the system KCI-AICI3, drawn 
from the data of references 26, 40, and 55. 




FIGURE 17. - The system NaF-Na 3 AIF 6 , drawn from 
the data of reference 14, and the system NaCI-Na 3 AlF 6 / 
drawn from the data of reference 29. 



Na + for the F~ ions to form more stable 
octahedral units than can be formed with 
the Cl~ ions; in any case, the observa- 
tion can be made that, in general, addi- 
tion of chlorides to fluorides causes 
greater decreases in liquidus tempera- 
tures than does addition of one fluoride 
to another fluoride. 

THE ALKALI CHL0R0ALUMINATES 

The ternary system NaCl-KCl-AlCl 3 has 
not been well studied because of the ex- 
perimental difficulties of working with 
very volatile, easily hydrolyzed samples. 
Barton (2_) presented some preliminary 
data, but the high-AlCl 3 portion of the 
system was not covered and the completed 
study was apparently never published. 
Midorikawa (40-41) determined the peri- 
tectics and eutectics in the systems 
NaCl-AlCl 3 and KCI-AICI3 and liquidus 
isotherms in the low-melting portion 
of the ternary system. Figure 18 (top) 
shows a hypothetical interpretation of 
the system, based on the ternary data of 
Midorikawa (41); the bottom presents the 
complete system, based on binary data 
shown in figures 13, 15, and 16 and on 
the data of Barton (2). It must be em- 
phasized that the diagram is hypothetical 
and that the equilibria in the high-AlCl 3 
portion of the system would be valid only 
at pressures >760 torr. The assumption 
has been made that the immiscible liq- 
uids observed in the binary systems also 
exist as ternary liquids above 193° C and 
that NaAlCl^ and KAICI^ form a complete 
solution series, at least just below sol- 
idus temperatures. This latter assump- 
tion is not in agreement with the work of 
Chikanov (9), who shows NaALCl I+ -KAlCl 4 as 
a simple binary system with a eutectic at 
125° C, -32 mol pet KA1C1 4 . 

MIXED ALKALI HALIDES 

Compositions containing all four alkali 
halides — NaCl , KCI, NaF , and KF — do not 
melt in accordance with the character- 
istics of a quaternary system. This is 
because NaCl and KF are unstable in com- 
bination; they react completely and 



22 



AICI, 



NaAICI 



KAICI. 




FIGURE 18. - The system NaCI-KCI-AICI 3 . Upper 
diagram redrawn from the data of reference 41. Lower 
diagram represents the system as inferred from the 
data of references 2, 19, and 41. 

Irreversibly at all temperatures to form 
NaF and KC1 , the combination of small 
cations and anions and of large cations 
and anions lowering the lattice energies 
because of the more favorable radius ra- 
tios. Phase equilibria can be shown, 
therefore, as two ternary diagrams with 
only one common binary, in this case NaF- 
KC1. Liquidus temperatures for the ter- 
naries were determined by Polyakov (47 ) , 
as shown in figure 19, and by Ishaque 
(25) , as shown in figure 20. Both au- 
thors indicated no solid solubility ex- 
cept for NaCl-KCl. Both diagrams show 
simple ternary eutectic relationships; 
the binary liquidus temperatures and eu- 
tectics are in reasonably good agreement 
with other authors' data. 




FIGURE 19. - The reciprocal system NaCI-KCI- 
NaF-KF, redrawn from the data of reference 47. 



NaF 
986° 



S80° 



NaCI 
801° 



X x N \ 






XI v \ 








\ \ 




X ' v 


\ \ 




x' \ 


\ \ 




A. \ 


\ \ 




900- \ \ 

\ \ 

\ I 

\ / 


\ \ 

\ 

\ 
\ 




\ / 


\ 




\ / 


\ 




X / 






\ / 
























~*^^ "*' >. 


) 










~--800---" \ 


/ 






/ 


6124- 




/ 






/ 






/ 






/ 






y/ 




\\ ~~~ 700 -" 






\ x. 


^«60° 




\ X. 






\ X. 






^ X*. 






\ — > 






\ .. / 












N \ \ 







KF 
860° 



KCI 
772° 



FIGURE 20. - The reciprocal system NaCl-KCl- 
NaF-KF, redrawn from the data of reference 25. 



23 



EQUILIBRIA IN THE SYSTEM 
NaCl-KCl-AlCl 3 -NaF-KF-AlF 3 

In addition to gaps in the information 
available about the system, there is con- 
siderable uncertainty about the composi- 
tions of the phases and their crystal- 
lization behavior. This is most obvious 
in the case of the binary relationships 
between Na 3 AlF 6 and K 3 A1F 6 . In 1962 Buk- 
halova (_7) , in the first detailed study 
within the system, reported that a com- 
plete solid solution exists between 
Na 3 AlF 6 and K 3 A1F 6 and that phases in 
that binary exist in stable equilibria 
with NaCl-KCl solid solutions. Bukha- 
lova's liquidus isotherms are shown in 
figure 21. No indication was given of 
the compositions of coexisting solid so- 
lutions, of the existence or nonexistence 
of a subsolidus mlscibility gap, or of 
the crystallographic parameters of the 
solid solution. The experimental method 
was "visual polythermal, " which consists 
of heating samples to a constant temper- 
ature and making a visual observation 
to determine if complete melting has 




FIGURE 21. - The system NaCI-KCI-Na 3 AIF 6 - 
K3AIF6, redrawn from the data of reference 8. 



occurred; if it has not, the temperature 
is increased and another observation is 
made, and this is continued until the 
sample appears to be completely liquid 
and the liquidus temperature is placed 
between the temperatures of the two final 
observations. 

Three years later, Bukhalova (7) and 
Mal^tsev ( 36 ) presented crystallization 
data for Na 3 AlFg and K 3 A1F 6 in equilib- 
rium with NaF, KF , NaCl , and KC1 , indi- 
cating that there is no solid solution 
between Na 3 AlF 6 and K 3 A1F 6 but that el- 
pasolite, K 2 NaAlF 6 , is present as a stoi- 
chiometric compound with a melting point 
of 932° C. To complicate the picture 
further, Edoyan (12) , in the same year, 
reported six stoichiometric compounds 
between Na 3 AlF 6 and K 3 AlF 6 :2Na 3 AlF 6 * 
K 3 A1F 6 ; 5Na 3 AlF 6 -3K 3 AlF 6 ; Na 3 AlF 6 »K 3 AlF 6 ; 
3Na 3 ALF 6 «5K 3 AlF 6 ; Na 3 AlF 6 «2K 3 AlF 6 ; and 
2Na 3 AlF 6 «5K 3 AlF 6 . Edoyan based the 
phase identification on the melting dia- 
gram and provided no characterization of 
the compounds. Edoyan also reported that 
addition of 14 to 15 wt pet K 3 A1F 6 low- 
ers the melting point of Na 3 AlFg from 
1,000° C to 832° C, almost 100° C below 
the lowest liquidus temperatures indi- 
cated by Mal'tsev (36). 

The data of Mal'tsev are shown in fig- 
ures 22-26. The crystallization volumes 
are described in terms of a compositional 
prism, the faces of which are shown 
by the three squares and two triangles 
of figure 22. Within the prism, they 
reported four stable crystallization 
tetrahedra: 

K 2 NaAlF 6 -Na 3 AlF 6 -Na 3 F 3 -Na 3 Cl 3 

K 2 NaAlF 6 -Na 3 Cl 3 -Na 3 F 3 -K 3 Cl 3 

K 2 NaAlF 6 -Na 3 F 3 -K 3 Cl 3 -K 3 F 3 

K 2 NaAlF 6 -K 3 AlF 6 -K 3 Cl 3 -K 3 F 3 

They also provided liquidus diagrams 
for four ternary sections: K 2 NaAlF 6 - 
Na 3 F 3 -K 3 Cl 3 (fig. 23); K 2 NaAlF 6 -Na 3 F 3 - 
Na 3 Cl 3 (fig. 24); K 2 NaAlF 6 -K 3 F 3 -Na 3 Cl 3 



24 



Na 3 AIF 
1.000' 



913° 



K 2 NaAIF, 
932° 
926° 



Na 3 AIF 



K 3 AIF, 
986° 




K 2 NaAIF 6 
926° 



K 3 AIF S 



FIGURE 22. - Liquidus surfaces of the faces of the compositional prism for the system NaCI-KCI-NaF-KF- 
t^^AIF^-r^AIF^, redrawn from the data of reference 36. 



(fig. 25); and K 2 NaAlF 6 -K3F3-K 3 Cl3 (fig. 
26) . None of the diagrams shows ternary 
equilibria, however. Figures 23, 24, and 
26 show primary crystallization fields 
for K 3 A1F 6 or Na 3 AlF 6 even though they 
were described as "tetrahedrating sec- 
tions." Figure 25 shows primary crystal- 
lization fields for both Na 3 AlF 6 and 
K 3 A1F 6 as well as for NaF. This indi- 
cates that the binary system KF-NaCl is 
not a true binary, and the existence of 
liquidus curves for NaCl and KF directly 
contradicts the data for that binary as 
shown in figure 22. It appears likely, 
therefore, that the primary crystalliza- 
tion fields are not well established and 



that the crystallizing phases were, in 
many cases, identified incorrectly. 

The "diagonal reciprocal" relationships 
shown in figure 22 appear to be question- 
able; the Alkemade lines between K^aAlFg 
and the alkali halides cross phase bound- 
aries in three of the four cases, with 
KC1, NaCl, and KF. This requires that 
elpasolite, K 2 NaAlF 6 , be a congruently 
melting phase in some sample compositions 
and an incongruently melting phase in 
others; this is possible only if elpaso- 
lite has a range of compositions. In 
view of the gently sloping liquidus sur- 
faces, shallow phase boundary troughs, 
and inadequate characterization of the 



25 





FIGURE 23. - The NaF-KCI-K 2 NaAIF 6 section through 
the compositional prism shown in figure 22. 



FIGURE 24. • The NaF-NaCI-K 2 NaAlF 6 section through 
the compositional prism shown in figure 22. 





FIGURE 25. - The KF-NaCI-K 2 NaAIF 6 section through 
the compositional prism shown in figure 22. 



FIGURE 26. - The KF-KCI-K 2 NaAlF 6 section through 
the compositional prism shown in figure 22. 



26 



crystalline phases, it is necessary to 
regard the data only as a general guide 
to aid in determination of temperatures 
of complete melting in the system. 

Figure 27 shows the binary system 
Na 3 AlF 6 -K3AlF 6 , constructed from liquidus 
temperatures taken from figure 22 and 
solid solution information presented 
later in this report. The upper portion 
depicts an interpretation of the data, 
plotted on the same scale as binary dia- 
grams shown earlier, to emphasize the 
flatness of the liquidus. The lower por- 
tion shows the same interpretation, ex- 
panded on the temperature axis to show 
the eutectics more clearly. The inset 
shows an alternative interpretation, with 
a peritectoid for the equilibrium among 
the limiting elpasolite solid solution, 
the limiting l^AlFg solid solution, and 
liquid. This interpretation could ac- 
count for incongruent melting of elpaso- 
lite in some cases. Some very meticulous 
experimental work would be necessary to 
determine relationships in this system. 

The system studied by Mal'tsev (36^) is 
not the complete system involved in for- 
mulation of aluminum fluxing salts be- 
cause Na3AlF 6 and K3AIF6 are not true 
components, being binary compounds of NaF 



yoo 




N..AIF 



40 so ao 
K.AIF.. mo) pel 



and AIF3 and of KF and A1F 6 , respective- 
ly. It provides no information about 
melting temperatures in cases where AIF3, 
AICI3, or other materials with Al-F ra- 
tios of <1:6 are added to the flux, for 
example. To provide a complete picture, 
the system must be defined as NaCl-KCl- 
AlCl 3 -NaF-KF-AlF 3 . Unfortunately, little 
information is available about this sys- 
tem other than that just described, which 
constitutes only a part of it. 

As a first effort to provide such in- 
formation, figure 28 shows a hypothetical 
ternary diagram of the system NaF-KF- 
ALF3 , constructed from the data of Buk- 
halova (36), which have been redrawn as a 
triaxial plot at the bottom, and from the 
data shown in figures 13, 15, and 16. To 
simplify the system and forego the uncer- 
tainty regarding the melting behavior of 
K 2 NaAlF 6 or solid solutions thereof, the 
750° C peritectic was changed to a eutec- 
tic in the subsystem K 2 NaAlF 6 -K 3 AlF 6 -KF; 



AIF, 



KAIF, 
574° 



Na,AI,F 




Na,AIF 
1,009' 



K,AIF, 
985° 



FIGURE 27. - The Na 3 AlF 6 -K 3 AlF 6 binary diagram, 
drawn from ternary diagram of reference 36. 



FIGURE 28. - The system NaF-KF-AIF 3 . Lower dia- 
gram represents data of reference 36, redrawn as a tri- 
axial graph. Upper diagram is hypothetical, based on 
binary equilibria data. 



27 



the phase boundaries within the subsystem 
K 2 NaAlFg-Na 3 AlFg-NaF were also changed to 
a more realistic, though hypothetical, 
configuration. Enough data are available 
to show that the Na 3 AlF 6 -K 3 AlF 6 binary 
represents an important dividing line and 
that addition of A1F 3 to compositions 
along that line causes a very sharp 
decrease in liquidus temperatures and 
ternary eutectics below the 560° C and 
694° C eutectics on the binaries. 

Two other ternary diagrams within the 
system are available. The system NaCl- 
NaF-AlF 3 (29), shown in figure 29, indi- 
cates that NaCl-Na 3 AlFg is a true binary 
and that addition of A1F 3 to mixtures of 
NaCl and Na 3 ALFg can lower the liquidus 
temperature to a 626° C eutectic. Figure 
30 shows that NaCl-KCl-NaF is a true 
ternary (51) . 

SUMMARY OF PHASE EQUILIBRIA DATA 

The melting behavior of compositions 
in salts containing NaCl, KC1 , A1C1 3 , 
NaF , KF, and A1F 3 has not been thor- 
oughly described in the literature; some 
generalizations are possible, however. 

PHASE RELATIONS IN THE SYSTEM 

Experimental research (5 7) has been 
carried out to address (1) the defini- 
tion of the correct system in terms of 



It is noted, for example, that crystal- 
line compounds with AlFg octahedral or 
alkali halide octahedral linkages are 
relatively stable, with high melting 
points and low vapor pressures, so that 
salt compositions within the volume 
bounded by NaCl, KC1 , NaF, Na 3 AlF 6 , and 
K 3 A1F 6 form reasonably stable melts. 
These compositions all have F-Al ratios 
>6. Addition of A1C1 3 or other phases 
with a F-Al ratio <6, however, lowers 
liquidus temperatures, drastically in- 
creases vapor pressures, and promotes hy- 
drolysis. Relative stabilities of the 
crystalline compounds in the system indi- 
cate that the Al 3+ ion is stable only in 
the AlFg octahedral configuration; in 
salts with F-Al ratios >6, the excess F + 
ions are bound strongly to Na + and K + 
ions, but in salts with F-Al ratios <6, 
the excess Al 3+ ions form highly volatile 
aluminum chloride species , predominantly 
the neutral dimer Al 2 Clg. In the system 
NaCl-KCl-ALCl 3 -NaF-KF-AlF 3 , therefore, 
the boundary between stable and unstable 
salt mixtures is the plane NaCl-KCl- 
Na 3 AlF 6 -AlFg. 

NaCl-KCl-AlCl 3 -NaF-KF-AlF 3 

subsolidus compatibility relationships 
and (2) the characterization of elpaso- 
lite in terms of its compositional range. 





FIGURE 29. - The system NaCI-NaF-AlF 3 , redrawn 
from the data of reference 29. 



FIGURE 30. - The system NaCl-KCl-NaF, redrawn 
from the data of reference 51. 



28 



EXPERIMENTAL PROCEDURES 

Reagent-grade NaCl , KC1 , NaF , KF, and 
A1F 3 were dried in air at 350° C for at 
least 24 h prior to weighing and mixing. 
Anhydrous AICI3 was used as received. 
All chemicals were obtained from MCB 
Chemicals, Cincinnati, OH. 6 All composi- 
tions were prepared in 50-g batches, with 
each component weighed to the nearest 
0.01 g, and were stored in airtight bot- 
tles. Only AICI3 and KF presented han- 
dling problems because of hydration in 
air. Special care was taken to weigh 
these components rapidly and to seal the 
samples immediately. For determination 
of subsolidus compatibility relation- 
ships, ~2-g samples of the compositions 
were sealed in evacuated borosilicate 
tubes and heated at 550°±10° C for 8 to 
24 h. After heating, the tubes were 
cooled and broken, and the reacted sam- 
ples were immediately pressed into pel- 
lets, using starch as a binder, and 
covered with plastic wrap to prevent ab- 
sorption of moisture. 

During the study, it was found that 
compositions with no free AICI3 or KF in 
the equilibrium assemblages could be 
melted in air without excessive vaporiza- 
tion losses and could be handled in air 
without moisture absorption, so several 
samples with the composition KAIF^ and 16 
samples on the Na 3 AlF 6 -K3AlF 6 join were 
prepared by melting in air, in porcelain 
crucibles, and quenched by pouring the 
liquids into a stainless steel beaker im- 
mersed in water. The quenched samples 
were ground with mortar and pestle and 
annealed in air at 800° C. 

Phases in the covered samples were 
identified by X-ray dif f ractometry , using 
CuKa radiation at a scanning rate of 1° 
20 per minute. Detailed X-ray data for 
KAIFlj and the quenched and annealed sam- 
ples on the Na 3 AlF 6 -K3AlF 6 join were ob- 
tained by scanning at 0.25° 20 per min- 
ute, using a quartz internal standard for 
precision measurement of diffraction 
angles. Intensities of the diffraction 



6 Reference to specific products does 
not imply endorsement by the Bureau of 
Mines . 



lines were measured by counting squares 
under the recorded diffraction peaks. 

SOLID SOLUBILITY IN THE 
ELPASOLITE PHASE 

The essentially identical structures of 
cryolite, elpasolite, and K-cryolite seem 
to be amenable to accommodation of vari- 
able amounts of cations of different 
sizes through slight distortions of the 
anion network and symmetry changes. This 
speculation, coupled with the importance 
of the cryolite series in crystallized 
fluxing salts, prompted a study of the 
solid solubility limits in the system 
Na 3 AlF 6 -K 3 AlF 6 . Detailed lattice and in- 
tensity measurements of the 16 compo- 
sitions quenched from the melts and an- 
nealed at 800° C are shown in figures 31 
and 32. The limited number of samples 
and the scatter in the lattice measure- 
ments do not permit accurate determina- 
tion of solubility limits, but the 
data show rather sharp changes in the 
parameters of cryolite and K-cryolite, 



8.440 



t 1 r 



1 r 



] 



8.430 - 
°< 

{2* 8.106 

z 

LU 

S 

UJ 
DC 

w 8.100 

< 

UJ 



KEY 
/\ Quenched 

O Annealed 



K-cryolite "a" ' 



UJ 

o 



8.094 



Elpasolite a 



A 



y 



2.756 - 



T^fi^yT 



Cryolite d(211) 




2.746 



1 

1 



J L 



50 
K3AIF., mol pet 



100 



FIGURE 31. - X-ray lattice measurements for sam- 
ples in the system Na 3 AlF 6 -r<3AlF 6 . 



29 




50 
K,AIF e , mol pet 



100 



FIGURE 32. - X-ray intensity ratio measurements 
for samples in the system t^^AIF^-t^AIF^. 



AlClj 




AICI, 



AIF, 



AIF, 



AIF, 



FIGURE 33. - Surfaces of the compositional prism 
for thesystemNaCI-KCI-AlCI 3 -NaF-KF-AIF 3/ show- 
ing subsolidus compatibility relationships. 



indicating mutual solubility of <5 mol 
pet in the end members. The intensity 
ratios provide a more reliable esti- 
mate of the solubility limits in elpaso- 
lite, whose compositional range is placed 
between 40±5 mol pet and 75±5 mol pet 
K^AlFg. The ratios for the annealed sam- 
ples are not significantly different from 
those of the quenched samples, so it is 
concluded that no appreciable unmixing 
occurs at 800° C or at room temperature. 

It might be expected that limited solu- 
bility occurs in the NaAlCl^-KAlCl^ se- 
ries and that there is limited solubility 
in KAIF^ and NasA^Fm, but these solu- 
bilities have not been determined. There 
is also a possibility of chloride-fluo- 
ride substitution, but this has not been 
studied. 

SUBSOLIDUS EQUILIBRIA 

To determine the order of the system 
NaCl-KCl-AlCl 3 -NaF-KF-AlF 3 , it was nec- 
essary to identify the irreversible ex- 
change reactions between mixed halides. 
X-ray analyses of reaction products 
formed in sealed evacuated tubes showed 
the following binary reactions: 



NaCl + KF + NaF + KC1 



(1) 



3 NaF + AICI3 ->• 3 NaCl + A1F 3 (2) 



3 KF + AICI3 ■»• 3 KC1 + AIF3 



(3) 



Reaction 1 verifies previous work (25, 
36, 47). Reactions 2 and 3 indicate re- 
ciprocal systems which, coupled with the 
chloride and fluoride ternaries, show 
that all phases and crystallization vol- 
umes can be represented graphically with- 
in a trigonal compositional prism whose 
faces are shown in figure 33. Once the 
reciprocal diagonals were established, 
the other tie lines were fixed by geomet- 
ric necesssity, as shown. 

The following binary reactions were 
also observed to occur in three component 
mixtures: 

KC1 + NaCl + AICI3 ->- NaCl + KAICI4 (4) 

NaF + 2KF + 2A1F 3 ->• A1F 3 + K 2 NaAlF 6 (5) 

Reaction 4 fixes the binary joins in the 
chloride ternary as NaAlCl^-KAlCl^ and 
NaCl-KAlCl^ , and reaction 5, coupled with 
previous work (13), fixes the binary 



30 



joins in the fluoride ternary as shown in 
figure 3. These were verified by subse- 
quent experimental determination of the 
resulting quaternary assemblages. 

To establish the binary joins within 
the compositional prism, the follow- 
ing ternary reactions were verified 
experimentally: 

2NaF + 3KF + A1F 3 ■+ NaF (6) 

+ KF + K 2 NaAlF 6 

5NaCl + 5KC1 + NaF + 2KF + A1F 3 (7) 

->• 4NaCl + 5KC1 + K 2 NaAlF 6 

KC1 + NaF + 4KF + A1F 3 -► KC1 (8) 

+ 2KF + K 2 NaAlF 6 



In addition, six compositions with dif- 
ferent percentages of NaCl , KC1 , and A1F 3 
were heated in sealed tubes and were 
found to undergo no net reaction. Reac- 
tions 6, 7, and 8 establish the exist- 
ence of stable joins between elpasolite, 
K 2 NaAlF 6 , and the component alkali ha- 
lides. This, coupled with the proven ex- 
istence of NaCl-KCl-AlF 3 as a stable ter- 
nary, requires that the compositional 
prism be divided into 12 compatibility 
tetrahedra, as shown in figure 34. To 
verify those conclusions, 12 composi- 
tions, one each within the 12 tetrahedra, 
were prepared and reacted in sealed evac- 
uated tubes. For ease of visualization, 
the prism has been divided into three 
volumes, as illustrated in figures 35-37, 
and- indicated below. The reactions ob- 
served follow: 



1. NaCl + 7KF + 2A1F 3 -»- KC1 + KF + K 3 A1F 6 + K 2 NaAlF 6 (9) 

2. 2NaCl + 5KF + A1F 3 ■»■ 2KC1 + NaF + KF + K 2 NaAlF 6 (10) 

3. 3KC1 + 4NaF + A1F 3 ->- 2NaCl + NaF + KC1 + K 2 NaAlF 6 (11) 

4. 2KC1 + 7NaF + 2A1F 3 ->- 2NaCl + NaF + Na 3 AlF 6 + K 2 NaAlF 5 (12) 

5. NaCl + 7KF + 3A1F 3 + KC1 + KA1F 4 + K 3 A1F 6 + K 2 NaAlF 6 (13) 

6. 2KC1 + UNaF + 5A1F 3 ■+ 2NaCl + Na 3 AlF 6 + Na 5 Al 3 F 11+ + K 2 NaAlF 6 (14) 

7. NaCl + 4NaF + 3A1F 3 ■*■ KC1 + A1F 3 + KAIF^ + K 2 NaAlF 6 (15) 

8. 2KC1 + 8NaF + 5A1F 3 -»• 2NaCl + A1F 3 + Na 5 Al 3 F 11+ + K 2 NaAlF 6 (16) 

9. NaCl + NaF + 2KF + 2A1F 3 + NaCl + KC1 + A1F 3 + K 2 NaAlF 6 (17) 

10. NaCl + 2KC1 + A1C1 3 + A1F 3 ->- NaCl + KC1 + A1F 3 + KA1C1 4 (18) 

11. 2NaCl + KC1 + 2A1C1 3 + A1F 3 ■*■ A1F 3 + NaCl + NaAlCl 4 + KAICI4 (19) 

12. NaCl + KC1 + 3A1C1 3 + A1F 3 + A1C1 3 + A1F 3 + NaAlCl,, + KA1C1 4 (20) 



As a further aid to visualization, fig- 
ure 38 shows the 12 tetrahedra in a sec- 
tion through the center of the prism; in 
this representation, points represent bi- the first time (57). 
nary assemblages, lines represent ternary 
assemblages, and triangles or parallelo- 
grams represent quaternary assemblages. 
The tetrahedra are numbered in accordance 
with the numbers of the reactions listed 
above. 



Tetrahedra 1-4 confirm the subsolidus 
compatibility reported by Mal'tsev (36) . 
Tetrahedra 5-12 have been determined for 



POWDER DIFFRACTION DATA 
FOR KAIF^ AND K 3 A1F 6 



There is some diagreement regarding 
the symmetry of KAIF^. Brosset (5_) 



31 



Na 3 CI 3 



Na 3 F 3 




K,CI, Na 3 CI 



K 3 CI 3 



Na 3 AIF, 



K,AIF R 



KAIF. 



AIF, 



FIGURE 34. - Subsolidus compatibility tetrahedra 
in the system NaCI-KCI-AICI 3 -NaF-KF-AIF 6 . 



Na,F. 




K,AIF t 



FIGURE 35. - Subsolidus compatibility in the por- 
tion of the system NaC I -KCI-AIC 1 3 -NaF-KF-A I F 3 coj^, 
responding to those in reference 36. 



Na 3 CI 




K 3 CI 3 



Na 3 AIF 



Na 5 AI 3 F,; 



K 3 AIF 6 



AIF, 



FIGURE 36. - Subsolidus compatibility tetrahedra 
in the volume bounded by NaCI-KCI-AIF 3 -Na 3 AIF6- 
K 3 AIF 6 . 



Na 3 CI 3 




K,CI, 



AIF 3 



FIGURE 37. - Subsolidus compatibility tetrahedra 
in the volume bounded by NaCI-KCI-AICI 3 -AIF 3 . 



32 



AICI 3 - AIF 3 



NaAICI 4 - AIF 



KAICI, - AIF3 




Na.CL - AIF 



3W13 *-»ii 3 



3CI3 - AIF3 



KAIF 4 - K3CI, 



Na 5 AI 3 F 14 - Na 3 CI 



Na 3 AIF 6 - Na 3 CI 3 



K 3 AIF 6 - K 3 CI 3 



Na,CI, - Na 3 F 



3" 3 



K,CI, - Na,F, 



K3CI3 - K3F3 



FIGURE 38. - Section through the compositional prism of the system NaCI-KCI-AlCl3-NaF-KF-AIF3 at the 
chloride-fluoride molar ratio of 1:1. 



originally assigned it to the tetragonal 
system and determined the structure on 
that basis. Phillips (44) , however, re- 
ported the symmetry as orthorhombic at 
low temperatures, with a reversible 
transition to cubic at about -15° C. 
Grjotheim (17) reported it to be tetra- 
gonal, with a transition to cubic at 
327° C. The powder data obtained in this 
study indicate tetragonal symmetry; be- 
cause the data in the Powder Diffraction 



file are incomplete, detailed data are 
listed in table 10, along with calculated 
intensities based on Brosset's structure. 
The calculated and observed intensities 
are in sufficiently good agreement to 
indicate that the reported structure is 
essentially correct. 

The tetragonal symmetry of ^AlFg at 
25° C as reported by Steward (59) , with 
unit cell parameters as in table 1, was 
confirmed by this study. 



CONCLUSIONS 



Available data on crystal structures, 
powder diffraction data, and phase 
equilibria data for the entire system 



NaCl-KCl-AlCl 3 -NaF-KF-AlF 3 have been as- 
sembled into one publication. Bureau 
of Mines determination of the subsolidus 



33 



TABLE 10. - Powder diffraction data for KAIF^ 



hkl 


Calculated 1 


PDF card 


2-595 


Observed 




d, A 


I 


d, A 


I 


d, A 


I 


001 


6.155 


8.1 


NR 


NR 


6.146 


9.8 


100 


3.570 


12.3 


3.57 


20 


3.570 


6.6 


101 

002 


3.088 
3.077 


71.1 
28.9 


| 3.08 


100 


3.084 


100.0 


110 


2.525 


28.4 


2.52 


50 


2.525 


27.4 


111 
102 


2.336 
2.331 


40.3 
23.4 


\ 2.32 


70+ 


2.333 


53.8 


003 


2.051 


4.6 


NR 


NR 


2.051 


5.5 


112 


1.952 


.1 


NR 


NR 


ND 


ND 


200 
103 


1.785 
1.779 


41.0 
34.8 


1.779 


100 


1.782 


82.3 


201 


1.714 


1.0 


NR 


NR 


ND 


ND 


210 


1.597 


1.7 


NR 


NR 


NM 


3.9 


113 


1.592 


.9 


NR 


NR 


ND 


ND 


211 

202 


1.546 
1.544 


16.9 
12.6 


| 1.540 


70 


I 1.542 


57.1 


004 


1.539 


22.0 


1.538 


50 


J 




212 


1.417 


5.5 


NR 


NR 


NM 


3.2 


104 


1.413 


.1 


NR 


NR 


ND 


ND 


203 


1.347 


3.1 


NR 


NR 


NM 


2.4 


114 


1.314 


3.7 


1.313 


20 


NM 


4.3 


220 
213 


1.262 
1.260 


10.1 
18.6 


> 1.258 


70 


1.260 


25.2 


221 


1.236 


.2 


NR 


NR 


ND 


ND 


005 


1.231 


<.l 


NR 


NR 


ND 


ND 


300 


1.190 


1.3 


NR 


NR 


ND 


ND 


301 
222 


1.168 
1.168 


3.1 
.2 


> 1.165 


50 


ND 


ND 


204 
105 


1.165 
1.164 


11.8 
1.3 


| 1.163 


50+ 


1.166 


15.7 


310 


1.129 


3.8 


NR 


NR 


NM 


5.6 



d = interplanar spacing; 1 
not measurable with precision; 

^pacings calculated for a 
calculated for z = 0.21, with 



= X-ray diffraction intensity; 

NR = not reported. 
= 3.570 A; c = 6.154 A at 22 c 
no temperature correction. 



ND = not detected; NM = 
C. Relative intensities 



compatibility relationships provides 
quidelines for systematic research into 
the properties of molten salts in the 
system. 

Based on information in the literature, 
the compatibility relationships described 
in this work, and observations made dur- 
ing the research, the system can be di- 
vided into three distinct crystallization 
volumes. Compositions within each of the 
volumes will melt to form molten salts 
with properties within ranges quite dif- 
ferent from those in the other volumes. 
Molten salts within the volume bounded by 
NaCl, KC1, NaF, KF , Na 3 AlF 6 , and K 3 A1F 6 



(fig. 35) are relatively stable, with 
high melting points and low vapor pres- 
sures; these salts have a F-Al ratio >6 
so that all the Al 3+ ions are in stable 
A1F 6 octahedra. Detailed property deter- 
minations are needed for a thorough un- 
derstanding of the relationships among 
salt composition, molten salt structures, 
properties, and fluxing efficiencies. 

Molten salts within the volume bounded 
by NaCl, KC1 , A1F 3 , Na 3 AlF 6 , and K 3 A1F 6 , 
shown in figure 36, are considerably less 
stable, with lower melting points and 
higher vapor pressures. The stable crys- 
talline phases within the volume are 



34 



NaCl, KC1, and all the fluorides of alu- 
minum and the alkalies; the F-Al ratio 
is <6 and, though the Al 3+ ion can be in 
octahedral coordination, the melting tem- 
peratures and other properties depend on 
the interactions of the alkali ions and 
the A1F 6 octahedra. 

Molten salts within the volume bounded 
by NaCl, KC1 , A1C1 3 , and A1F 3 , shown in 
figure 37, crystallize to form only one 
stable fluoride, A1F 3 , which is a vola- 
tile phase, along with the even more vol- 
atile phases A1C1 3 , NaAlCl^ , and KAICI^. 
The F-Al ratio is <3, and the Al 3+ ions 
necessarily are forced into unstable co- 
ordination with the halide ions. 



Very little information on molten salt 
properties within the volumes shown in 
figures 36 and 37 is available in the 
literature. In fact, density, vapor 
pressure, surface tension, and viscosity 
data are virtually nonexistent except for 
some of the binary assemblages. 

Because deliberate additions are com- 
monly made to aluminum fluxing salts, 
differential vaporization and hydrolysis 
reactions can change salt compositions 
during use, and reactions with components 
of the scrap can alter chemistry, it is 
essential that the entire system be char- 
acterized in terms of properties. 



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37 



HI 



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